Because of its extreme thinness and elaborateness, a molecular assembly such as a monomolecular film (monolayer) which has a molecular arrangement or a built-up film (monolayers, i.e., multilayer) that comprises laminating a plurality of monolayers is quite versatile and has uses in applications as a material for electronics device use and as a material for surface protection use, as well as an ultrafiltration film, a thin film for sensor use as a permeation controlling film for material delivery relying on the selective permeability of gaseous molecules and ions of the molecular assembly.
A phospholipid is a main component of a cell biomembrane. In a biomembrane, a phospholipid forms a double layer structure having a bimolecular structure and performs many functions which are essential for various vital processes. In addition, it is known that, when either of natural and artificial phospholipids is dispersed in water, it forms a closed vesicle, called a liposome, which is made of a bimolecular membrane which is molecularly oriented. Attempts have been made to apply liposomes to drug delivery and artificial blood systems in medical science and pharmacology, as well as to production of artificial cells and microcapsules. In addition to the above-described applications as a monolayer and a multiple layer, application of the molecular assembly to medical materials has also been attempted due to the biocompatibility of liposomes.
The Langmuir-Blodgett (LB) method is commonly known as a method for forming an monomolecular film at a gas-liquid interface and building up the films on a support material. Recently, the use of various LB film prepared with this method has been expending in many field as organic ultrathin films (cf. Solid Physics (Japanese), vol. 17, No. 12, p. 45 (1982)).
Although the molecular assemblies including LB films and liposomes can be used to perform various functions based on the orientation of molecules and extreme thinness, they have disadvantages because the film can be easily destroyed due to its physically delicate structure and, in some cases, a less elaborate structure due to a structural defect in the film depending on its composition. As a result, preparation of films which are more elaborate by physically strengthening the film structure of these molecular assemblies is a subject to be studied.
One effective means to physically strengthen the film structure of these molecular assemblies is cross linking or polymerization of molecules. With regard to the polymerization of molecular assemblies such as LB films and liposomes, commonly used polymerizable compounds and polymerization patterns have been summarized by H. Bader et al. in Advances in Polymer Science, vol. 64, p. 1 (1985) and by R. Buschl et al. in Macromol. Chem. Suppl., vol. 6, p. 245 (1984).
Studies on polymerizable amphipathic compounds became active in 1980s. In the early stage of these studies, unsaturated vinyl, diene and diacetylene derivatives were used as the polymerizable compound and the polymerization was performed by cleaving unsaturated bonds in the compound with the aid of ultraviolet (UV) rays or radiant rays such as .gamma.-rays. Although the rigidity of the films was improved by these polymerization methods in most cases, it was difficult to maintain the order of molecular arrangement because of strains caused by the polymerization. As has been indicated in Macromolecule (edited by A. Laschewsky and H. Ringsdorf; vol. 21, p. 1936, 1988), only a small number of compounds can result in a polymer film having an excellent order of molecular arrangement because the orientation of a film is affected greatly by the length of the alkyl chain and the kind of terminal hydrophilic groups.
A. Laschewsky et al. have disclosed in J. Am. Chem. Soc., vol. 109, p. 788 (1987) that support of polymerization groups via spacer groups is necessary for the maintenance of the order of molecular arrangement order in the case of amphipathic compounds having various unsaturated bonds useful for the radiation polymerization and the like. Also, JP-A-57-159506 (the term "JP-A" as used herein means an "examined Japanese patent publication") discloses an example of the application of monomolecular and built-up polymer films to ultrafiltration films, by preparing these polymer films from an unsaturated compound (a surface active agent) using radiation polymerization.
These compounds with unsaturated bonds cause the following disadvantageous problems when they are subjected to the prior art radiation polymerization process. Firstly, since disordered structural arrangement and a random aggregation and deposition of molecules tend to occur in such a polymerization process, it is necessary to introduce a special molecular design such as insertion of spacer groups in order to prevent these problems. Secondly, application of ultraviolet rays or .gamma.-rays results in the decomposition and denaturation of various additive which are frequently present with polymerizable amphipatic molecules. Thirdly, the biocompatibility of films obtained with this type of polymerization is very poor in general, resulting in the limitation of their application to living tissues as a film for use in the permeation control of drugs and the like and other films for biological use.
Consequently, techniques other than radiation polymerization, such as the formation of disulfide bonds by means of oxidation polymerization of dithiole, have been proposed for example in J. Am. Chem. Soc., vol. 109, p 4419 (1987). Radical polymerization of the foregoing compounds with unsaturated bonds in the presence of an initiator may also be useful. In the technique, however, the use of an initiator is essential at the time of the polymerization. Therefore, not only is a process for the removal of the initiator from the film system after completion of the polymerization is required, but also the effect of the initiator e.g., as an oxidation-reduction agent, on substances also present is a problem.
Examples in which the biocompatibility of film was improved by a modification of polymerization systems include methods for self-condensation polymerization of molecular films composed of long chain alkyl derivatives of amino acids have been proposed in Macromol. Chem. Rapid Commun., vol. 3, p. 167 (1982) and in Thin Solid Films, vol. 133, p. 39 (1985) and a method for condensation polymerization of similar derivatives using carbodiimide as a condensation agent has been proposed in J. Am. Chem. Soc., vol. 108, p. 487 (1986). These methods, however, have certain disadvantages such as an extremely slow condensation reaction in the case of a self-condensation polymerization and residual condensation agent and by-products where a condensation agent is used.
Methods for polymerizing a diester phosphate monomer derived from a methacrylic acid ester in the presence of a polymerization initiator and methods for connecting a phospholipid analogue with the side chains of a synthetic polypeptide have been proposed in, .for example, Makromol. Chem., vol. 178, p. 2963 (1977), Makromol. Chem., vol. 179, p. 2349 (1978), Makromol. Chem. Rapid Commun., vol. 6, p. 285 (1985) and J. Makromol. Sci. Chem., vol. A25, p. 115 (1988) to prepare analogous biocompatible phospholipid films. By these methods, however, it is difficult to obtain a film which has a similar molecular orientation and arrangement to those of a natural biological membrane using these methods. In addition, the structure of these phospholipid analogous differs greatly from that of the phospholipid forming the biomembrane.